1. Field of the Invention
The present invention relates to a synchronized rectifier type DC-DC converter and, in particular, relates a voltage step down type DC-DC converter which uses, for example, PWM operation and includes a reverse flow preventing means of such as switching elements and circulating use elements without detecting current value of a reactor, and a control circuit therefor.
2. Conventional Art
A voltage step down synchronized rectifier type DC-DC converter is used as a power source for a variety of information equipment. A variety of techniques for improving conversion efficiency of such devices during light load have been carried out. JP-A-11-235022 (1999) discloses a synchronized rectifier circuit which prevents a reverse flow of reactor current. As shown in FIG. 11, the circuit is constituted by a switching element 51, a circulation use switching element 52, a diode 53, a reactor 3, a capacitor 4, a reactor current detection circuit 501 and a control circuit 502. The reactor current detection circuit 501 monitors the reactor current, and when a tendency toward reverse current is detected, it commands the control circuit 502 to interrupt the circulation use switching element 52. Further, the reactor current detection circuit 501 comprises, for example, a resistor connected in series with the reactor.
In the conventional power source the reactor current is monitored and when a reverse current flow tends to occur, the circulation use switching element 52 is interrupted to prevent a reverse flow. In this manner, diminution of conversion efficiency under a light load condition may be prevented; however, the circuit requires a detection circuit such as a resistor connected in series with the reactor 3. Further, in the conventional power source only a reverse flow of the reactor current is prevented, but the current value of the reactor is not controlled; therefore, a ripple in the output voltage during a light load can not be reduced.
An object of the present invention is to provide a DC-DC converter and a control circuit which controls the DC-DC converter with a high efficiency and a low ripple during light load conditions without detecting the reactor current or an equivalent thereof, and realizes a high response performance during a load variation.
FIG. 1 shows the structure of a converter according to a first aspect of the present invention, in which main conversion circuit includes a switching element 1, a circulation use switching element 2, a reactor (inductance) 3, a capacitor 4 and an output current detector 5. This is a synchronized rectifier and voltage step down type converter which converts an input DC voltage to a predetermined output value. A control circuit 200 as shown in FIG. 1 is provided with a calculation circuit which calculates the reactor current from detected values of output voltage and current, and controls the converter operation with the calculated value of the reactor current. According to a first aspect of the present invention, the switching element 1 and the circulation use switching element 2 in the converter can be properly controlled without detecting the reactor current (or equivalent thereof) such as with the reactor and the circulation use switching element. In this manner, the conversion efficiency can be enhanced and the ripple can be reduced.
FIG. 2 shows the structure of a converter according to a second aspect to the present invention. A control circuit 200a in FIG. 2 is provided with a calculation circuit which calculates a reactor current and a capacitor internal voltage from detected values of the output voltage and current, and controls the converter operation with the calculated value of the reactor current and the calculated value of the capacitor internal voltage. Further, frequency control is performed, which provides an xe2x80x9conxe2x80x9d interval for controlling the calculated value of the reactor current in relation to the output current, and an xe2x80x9coffxe2x80x9d interval for controlling the calculated value of the capacitor internal voltage in relation to a reference potential. According to the structure of the second aspect of the present invention, ripple effects can be reduced during a light load condition and an efficiency enhancement can be realized.